The present invention relates to a cathode structure being an electron emitting source, and an electron gun adopting the same, for a cathode ray tube (CRT).
In general, a CRT is provided with a panel 100 having a fluorescent film 400 formed on the inner surface thereof, and a funnel 200 which is attached to the panel 100 and includes a neck portion 201 having an electron gun 300 installed therein, and a coned portion 202 having a deflection yoke 203 installed there around, as shown in FIG. 1. Reference numeral 301 denotes a support for supporting the electron gun 300.
In the CRT as constituted above, an electron beam emitted from the electron gun 300 is deflected by the deflection yoke 203 and projected onto the fluorescent film 400, thereby exciting a fluorescent material thereon to form an image.
The structure of the electron gun 300 installed in the neck portion 201 of the funnel 200 for emitting electrons is shown in FIG. 2. The electron gun 300 has a glass bead 11, a cathode structure 310 fixed to the bead glasses 11 for emitting an electron beam, a control electrode 14, a screen electrode 15, and a focus electrode 16 and a final accelerating electrode 17 which are sequentially installed from the screen electrodes 15 and form an electron lens.
Electrons are emitted from the cathode structure 310 by applying a predetermined voltage to each electrode and the cathode structure 310 of the thus-constituted electron gun 300, and electron lenses are formed between adjacent electrodes 14-17. Therefore, the electron beam emitted from the cathode structure 310 is focused and accelerated as it passes through the electron lenses.
The time required for normally emitting the electron beam from the electron gun 300 and the current density of the electron beam are determined by the cathode structure 310. The structure of the cathode structure 310 is shown in greater detail in FIG. 3. The cathode structure 310 has three cathode assemblies 12, an external case 13 fixed to the control electrodes 14 of FIG. 2, and an insulating body 28 within the external case 13 and through which the three cathode assemblies 12 are disposed.
Each cathode assembly 12 includes a cylindrical sleeve 21, a cap member 22 fixed to the end portion of the sleeve 21, an electron emitting material layer 23 coated on the upper surface of the cap member 22, and a holder 25 for supporting the lower part of the sleeve 21. Each cathode assembly 12 is fixed inside an internal case 26, and the insulating body 28 isolates the external case 13 from the internal case 26. In addition, the cathode assemblies 12 of FIG. 3 are of the indirect heating type, and a heater 24 is inserted in the sleeve 21. On the other hand, in a direct heating type cathode structure, a heater is directly connected to a cathode.
The heater 24 emits heat when a predetermined potential is applied to the heater 24 of the thus-emitted cathode structure 310. Heat emitted from the heater 24 is transferred to, thereby heating, the cap member 22, the sleeve 21, and the holder 25. If the cap member 22 is heated to, for example, about 800.degree. C., electrons are emitted from the electron emitting material layer 23 coated on the upper surface of the cap member 22.
There are two types of electron emitting material layers: one is an oxide type obtained by coating a material containing an oxide of an alkali earth metal as a major component on a base metal containing a reducing material; and the other is an impregnation type obtained by impregnating an electron emitting material in the pores of a porous metal.
However, in a conventional cathode structure, since an electron emitting material is heated after a cap member and a sleeve are heated by a heater, a large amount of time is required for normal emission of electrons. Therefore, an extended period of time, generally 8-9 seconds, is needed to form an image on a screen of a CRT.
In addition, the cap member and the sleeve of the cathode structure are heated and then thermally expanded by the heater when they are operated, a convergence drift and a thermal drift in which the positions of electron beam shifts are generated. To prevent these drifts, electron beam emission aging should be performed during a manufacturing process, in which a cathode is heated for a long time so that the cathode has a physical and chemical structure more likely to emit electrons accurately. As a result, product yield is lowered.
In the case of a color CRT which excites red, blue, and green fluorescent material, a stabilization time for white balance is long due to the temporal differences among the initial thermal expansions of the cathode structure, the control electrodes, and the screen electrodes.
Furthermore, the conventional cathode structure consumes electric power, for example, 2-4 W, to heat the electron emitting material layer, and exhibits an electron emission characteristics distribution due to non-uniform heat transmission with respect to each of red, blue, and green cathodes.
In addition, since the heater and its support are included in the electron gun, the total length of the electron gun becomes large and a manufacturing process of the heater is very complicated, resulting in a decrease in product yield and an increase in failure rate.